The present invention relates to a process for ultrasonic bonding, more particularly adapted for the assembly of elements at least one of which is impregnated with a resin.
Bonding techniques are widely used in the field of composites, particularly in the processes for production of acoustically absorptive panels, and more particularly in the production of resistive layers.
According to the teaching of French patent application FR-01.08036 in the name of the applicant, a resistive layer comprises a first acoustically damping element in the form of a porous surface, for example of an acoustic metallic cloth and at least one second structurally reinforcing element in the form for example of fibers pre-impregnated with a thermosetting or thermoplastic resin or a mixture of resins. Generally, the resistive layer is made from multi-layer strips comprising acoustically damping elements and structural reinforcement.
The connection by bonding using one or several resins must guarantee not only a solid mechanical connection between the different elements, but also keep the quantity of opening of the acoustically damping element in the present case.
To obtain optimum mechanical properties, the connection by bonding must be carried out at a suitable temperature and pressure as a function of the resin used.
One of the techniques used to carry out this bonding consists in using a nitrogen torch blowing hot nitrogen to supply the quantity of heat necessary to use the resin and a pressure roller to apply pressure on the layers for using the resin.
This technique is not satisfactory for the following reasons.
First of all, the heating and pressure localities are offset, which necessarily leads to cooling of the resin, such that at the moment of its use, during pressure, the latter is not at the required temperature, inevitably changing the characteristics of adhesion and hence the quality of the connection. To compensate for this cooling due to offset, it has been practiced to superheat the material at the risk of burning the elements or the resin and making it lose its adhesive properties.
Then, heating is carried out in a surface manner, such that it is necessary to overheat the material of one of the layer so as to obtain good heat penetration.
Finally, given the numerous parameters involved, the monitoring of the heating temperature of the resin is almost impossible, so that it is very difficult to obtain a constant and reproducible quality of bond. Also, this technique cannot be used for industrial type fabrication, particularly to ensure the connection of strips together when these latter are disposed on a mold.
Also, the present invention seeks to overcome the drawbacks of the prior art by providing a bonding process for at least two elements of which at least one is impregnated with a resin or a mixture of resins, permitting optimum use of the resin in a reproducible manner, while limiting the risk of change by overheating of the materials of the connected layers but also the resin, said use being adapted easily to be integrated into an industrial process.
To this end, the invention has for its object a process for bonding at least two elements of which at least one is impregnated with a resin or a mixture of thermostable thermoplastic resins, characterized in that the elements are placed in contact by pressure means which are also adapted to transmit vibrations to at least one of the elements so as to obtain a temperature rise by friction at the interface, to the operating temperature comprised at a shoulder, resulting in a less rapid increase of temperature, corresponding to passage from the solid state to the liquid state of the resin or of the mixture of resins.
Preferably, when one of the elements has openings, the operating temperature is selected at the beginning of the shoulder.
According to another characteristic, to bond several elements using several types of resins or mixtures of resins, it is preferable to carry out the bonding in several steps as a function of the number of types of resins or mixtures of resins, so as to adjust the operating temperature to each type.
According to a preferred embodiment, the pressure means for transmitting vibrations comprise a sonotrode head which is adapted to transit vibrations and an adjustable pressure force and to press the elements against a support element.
As a function of the use, the elements to be bonded move below the sonotrode head so that this latter moves relative to said elements.